The foundations holding up the damaged Bear Cut Bridge appear to have fewer years left in them than Miami-Dade engineers expected, which could accelerate the county’s plans to build a brand-new structure.

The 67-year-old bridge connecting Virginia Key to Key Biscayne has a remaining lifespan of about 20 to 30 years, an analysis of the pilings holding up the structure concluded — not the 40 to 50 years public works administrators had estimated.

The shorter life expectancy should not affect repairs already under way along the 1,200-foot bridge to replace corroded steel beams holding up the roadway.

But it will likely force Miami-Dade to begin planning in earnest for a new bridge once the year-long repairs are completed, interim county engineer Antonio Cotarelo told a group of county commissioners Tuesday.

“That is a very long process,” he advised the infrastructure and capital improvements committee. “We’re planning to start as soon as this project is done.”

Mayor Carlos Gimenez had said the repairs would not prevent the county from exploring the new bridge option. The shortened lifespan appears to have crystallized his administration’s position from merely considering a new bridge to expecting to build one — though Miami-Dade not yet secured funding.

The county’s public works and waste management department, which oversees bridges, has predicted that building a new bridge similar to the existing one would take seven to 10 years in planning, designing, permitting and construction.

The project would cost about $100 million. A hired consultant pegged the price at about $59 million, but preliminary studies, insurance and other costs would add about $40 million, the county said.

The Key Biscayne Village Council had formally asked Miami-Dade to forgo the $31 million bridge repairs in favor of cheaper, temporary reinforcement known as bracing in order to build a new bridge more quickly. County engineers feared bracing might not last long enough for a new bridge to be built, putting the only road in and out of Key Biscayne at risk.

The smaller West Bridge, immediately after the Rickenbacker Causeway toll plaza, is undergoing similar repairs. The westbound portions of both bridges were built before the eastbound portions, which were constructed in the 1980s and are structurally sound.

When commissioners approved the expedited repairs, the consultant hired to review the extent of the Bear Cut Bridge damage had said the 1944 structure’s life expectancy could not be determined because no documentation provided details of the foundations’ construction.

Commissioners asked for an expensive technical study to analyze the underground bridge pilings.

Kiewit Infrastructure, the firm performing the repairs, hired the Sunrise-based Hardesty & Hanover firm to evaluate eight randomly selected pilings out of 440 — less than two percent, but a representative sample across different bridge spans that the county could afford, Cotarelo said. Such detailed a study is not typically performed for a bridge rehabilitation project.

The nearly $486,000 study found that with proper maintenance and barring major coastal surge, wind or a boat crashing into it, the bridge has a life expectancy of 20 to 30 more years.

“As concrete structures age in severe environments (sea spray and salt air contamination) more maintenance is required to ensure substructure integrity,” Timothy J. Noles, a principal for Hardesty & Hanover, wrote in the report to Kiewit.

As long as the pilings don’t have to bear any additional weight, they meet the safety standards for a bridge built in the 1940s, the study concluded. The repairs are not expected to increase the load on the pilings.

The study examined the length of the pilings and how deeply embedded they are in the sea bed. Two of the eight pilings did not meet an established safety threshold.

Yes, the engineer responded. But that doesn’t mean the bridge is unsafe.

“We don’t see cracks” on the pilings, Cotarelo said. “We don’t see anything to indicate there is any issue.”

At a meeting with county engineers Tuesday night, Key Biscayne Council member Michael Kelly noted that while the existing bridge’s condition may be acceptable, it is not optimal — and won’t be until a new bridge is built to replace it.

“We’re going to spend all this money for a bridge that’s going to take care of the short-term need, but we’re really only going to get something, at the end of the day, that’s acceptable,” he said. “And we’re going to have to wait 10 years before we get a new bridge.”

Yet being on the same page that a new bridge is needed is a step forward, said Key Biscayne Vice Mayor Mayra Peña Lindsay. Key Biscayne’s own engineering consultant must still review the county study.